The present invention relates to a passive optical network system (hereinafter referred to as “PON”) in which a plurality of subscriber connection devices share an optical transmission line.
A PON generally consists of an optical line terminator (hereinafter referred to as “OLT”) and a plurality of optical network units (hereinafter referred to as “ONU”). In the PON, signals from terminals connected to the ONUs, such as personal computers (hereinafter referred to as “PC”) or IP phones, are converted into optical signals, and the optical signals are transmitted to the OLT through branch optical fibers. The branch optical fibers from the ONUs are coupled by an optical splitter. The optical signals are optically multiplexed (by time division) on a trunk optical fiber connected to the optical splitter. Thus, the optical signals reach the OLT.
Each of the plurality of ONUs may be installed in any position as long as the distance between the OLT and the ONU satisfies the range defined in Chs. 8 and 9 of Recommendation G984.1 by the International Telecommunication Union (ITU-T). In other words, due to a variation in the transmission delay of an optical signal between the OLT and each ONU, there is a possibility that the optical signals outputted from the ONUs collide and interfere with each other on the trunk optical fiber if the transmission delay is not taken into consideration. For this reason, by using ranging technology defined in Ch. 10 of ITU-T Recommendation G984.3, the delays in the output signals of the ONUs are regulated so that the ONUs can look as if they had been set at an equal distance from the OLT. Further, by using dynamic bandwidth allocation (hereinafter referred to as “DBA”) technology defined in ITU-T Recommendation 983.4, the OLT allocates bandwidths to as many ONUs as possible in accordance with requests from individual users so that each ONU can transmit a signal in the allocated bandwidth in timing defined by an instruction (hereinafter referred to as “grant instruction” or simply “grant”) from the OLT. Thus, optical signals from the ONUs to the OLT can be prevented from colliding or interfering with each other on the trunk optical fiber.
A signal transmitted from each ONU to the OLT is referred to as “upstream signal”. The upstream signal is a variable-length packet (hereinafter also referred to as “packet” simply) which consists of an overhead (which is a fixed-length overhead also referred to as “burst overhead”) and burst data. The overhead includes a preamble and a delimiter. The burst data includes a variable-length payload signal. A guard time is set immediately before each packet so as to prevent the packet from chiding with any packet transmitted earlier. On the other hand, a signal transmitted from the OLT to each ONU is referred to as “downstream signal”. The downstream signal is a 125-microsecond frame signal which consists of a frame synchronization pattern, a PLOAM, a grant instruction and a frame payload.
The grant instruction from the OLT using an area referred to as “US Bandwidth MAP” assigns an upstream signal transmission grant timing for each ONU. The grant instruction includes a Start value designating a start of transmission of an upstream signal in each ONU, and an End value designating an end of the transmission. The designation of each value is made in byte units. A non-signal area lies between the End value and a Start value of the next upstream signal. The non-signal area corresponds to the aforementioned guard time. A plurality of bandwidth allocation units called T-CONT (Transmission CONTainer) can be assigned to each ONU. For example, bandwidth allocation can be made for each service accommodated in one and the same ONU. Such a Start value and such an End value are assigned for each T-CONT. These values are also referred to as “grant values”.
In a PON called a GPON defined in the aforementioned recommendation, a variable-length packet called a GEM (GPON Encapsulation Method) packet is transmitted from each ONU to the OLT. A 5-byte header called a GEM header is attached to the GEM packet. A length of the transmission signal, a flow label, and a flag indicating the presence/absence of processing called fragmentation which will be described later, are contained in the GEM header.
According to the aforementioned ITU-T recommendation, the OLT gives grant instructions to each ONU with a period of 125 microseconds. That is, the OLT transmits grants to specified ONUs with the 125-microsecond period, and the specified ONUs transmit data to the OLT while sharing the optical fiber by time division in accordance with the grants. If the OLT performs the aforementioned DBA with the 125-microsecond period, all the ONUs (or all the T-CONTs) can be notified of bandwidths assigned to themselves respectively directly as grant values by grants, and all the ONUs can output signals in accordance with the grants respectively. However, DBA in a real PON is not performed with the 125-microsecond period, but is mostly designed to be performed with a period longer than a period designated by a grant instruction, for example, with a period of 0.5 miliseconds or 1 milisecond for the following reason. That is, microsecond-order response time is not required in data communication such as Internet access. Accordingly, satisfactory bandwidth allocation can be secured for operation even if a processor having a low throughput of about several tens of MIPS spends a time of about 0.5 miliseconds or 1 milisecond on DBA which is high in calculation cost. That is, DBA may be performed with a period longer than a grant period.
The OLT decides data lengths for all the ONUs (T-CONTs) in DBA within a period of a multiframe of about 4 to 8 frames extending over 125-microsecond frame boundaries. The ONUs are allowed to transmit data with the decided lengths respectively. Therefore, on giving a grant from the OLT to each ONU, the OLT performs processing to divide the data length decided in DBA into a plurality of 125-microsecond frames and designate the data length as grant values in the respective frames. That is, a grant value for an ONU may be allocated over a frame boundary. Processing in this case is carried out by a mechanism called fragmentation defined in Ch. 8.3.2 of ITU-T Recommendation G984.3 so as to divide a signal from the ONU into two frames (or to generate another grant instruction).
According to the same recommendation, when the OLT divides data length allocated to an ONU (T-CONT) into a plurality of 125-microsecond frames in DBA, a GEM header must be also attached to each granted packet in the divided frames. In original DBA, data length is allocated to each ONU by the OLT while the length of one (5-byte) GEM header which must be attached to the head of any packet is taken into consideration as described previously. However, the length of GEM headers which must be attached to the second and later ones of the divided packets is not taken into consideration. Therefore, when a grant given to an ONU is divided over a plurality of 125-microsecond frames, the data volume of a signal the ONU could have transmitted originally is reduced by GEM headers attached to packets received in the second and later 125-microsecond frames according to the recommendation.
For example, assume that up to 64 ONUs are connected to the OLT, and 32 VoIP services are accommodated in each ONU. When dynamic bandwidth allocation (DBA) is performed with the aforementioned T-CONTs being associated with the individual VoIP services, the OLT should execute DBA for the VoIP services upon 2,048 T-CONTs individually. Here, assume that each VoIP service requires an upstream communication bandwidth of 256 kbit/sec. When the OLT executes DBA with a period of 0.5 miliseconds on this assumption, the OLT must provide 256 kbit/sec×0.5 miliseconds/8=16 bytes for each T-CONT. In addition, the OLT should execute DBA in consideration of one (5-byte) GEM header which is required to be attached to the head of data. Therefore, the OLT should give each T-CONT a grant to transmit data of 16+5=21 bytes. To transmit data of 21 bytes every 0.5 miliseconds means each T-CONT will consume a bandwidth of 21×8/0.5 miliseconds/1000=336 kbit/sec. When the 2,048 VoIP services are in use simultaneously, a bandwidth of 336 kbit/sec×2048=688.128 Mbit/sec is consumed in the PON as a whole.
If the upstream signal rate from each ONU to the OLT is 1.24416 Gbit/sec (hereinafter often rounded to 1.2 Gbit/sec), the volume of data which can be transmitted in 125 microseconds corresponding to the grant instruction period can be expressed by 1.24416 Gbit/sec×125 microseconds/8=19440 bytes. The aforementioned data of 21 bytes can be indeed put in the data length of 19,440 bytes corresponding to one grant instruction period. However, when the data of 21 bytes are laid over the boundary between frames of 125 microseconds corresponding to the grant instruction period unfortunately, a phenomenon that the data of 21 bytes are divided into two upstream frames will occur due to the mechanism called fragmentation. According to the mechanism, for example, the data of 21 bytes may be divided into 15 bytes in the first frame and 6 bytes in the next frame. In this case, a packet of the first 15 bytes consists of a GEM header of 5 bytes and a payload of 10 bytes, while a packet of the next 6 bytes consists of a GEM header of 5 bytes, which must be attached due to the division according to the recommendation, and a payload of 1 byte. Thus, in spite of a signal length of 21 bytes allocated by DBA, a payload part of only 10+1=11 bytes can be transmitted substantially. Data of about 31% to the data of 16 bytes satisfying the required bandwidth of 256 kbit/sec will be untransmitted, thereby causing great influence on the communication quality.
The layout of a signal from each ONU (T-CONT) on an upstream frame differs from one DBA period to another DBA period. It is therefore impossible to know in advance whether a specific upstream signal will or not lie over boundaries among a plurality of 125-microsecond frames so as to cause fragmentation. If it is assumed that there is a possibility that fragmentation occurs in data from all the T-CONTs, redundant grant length may be allocated to all the T-CONT signals in view of 5-byte GEM headers which should be attached additionally. In such a manner, even if fragmentation occurs, untransmitted data as described previously will not appear. Thus, deterioration of communication quality can be avoided. However, a bandwidth required therefor will reach (21+5)×8/0.5 miliseconds/1000=416 kbit/sec per T-CONT. A bandwidth of 416 kit/sec×2048=851.986 Mbit/sec will be consumed in the PON as a whole, thereby resulting in a large useless bandwidth.